Elastomeric polymers such as styrene-butadiene rubbers having a styrene content of from about 20 percent to about 35 percent are commonly produced in inert organic solvents such as hexane. These polymers can be terminated by a number of different compounds including silane containing compounds to yield silane end-capped polymers. This siloxane termination can also result in an increase in the Mooney viscosity of the treated polymer similar to the increase that occurs during tin coupling. However, upon subsequent desolventization of the siloxane-terminated polymer through the use of either steam or heated water, an even larger increase in Mooney viscosity often occurs during the hydrolysis of siloxane groups such as pendant --SiOR groups on the siloxane end groups thereby leading to coupling of the polymer via formation of Si--O--bonds between two siloxane end groups.
Thus, a process utilizing steam or heated water in the desolventization of siloxane end-capped polymers containing hydrolyzable groups such as pendant --SiOR groups has always been accompanied by an increase in the Mooney viscosity of the polymers due to hydrolysis and subsequent coupling that occurs between the terminal siloxane groups.
Various attempts have been made to overcome this hydrolysis and coupling problem. For example, U.S. Pat. No. 5,659,056 discloses the use of acids such as C.sub.1 to C.sub.12 aliphatic and C.sub.6 to C.sub.12 cycloaliphatic and aromatic carboxylic acids including acetic acid, propionic acid, butyric acid, decanoic acid, cyclohexanoic acid, benzoic acid and the like, as well as acyl halides thereof, as viscosity stabilizing agents to treat the polymer prior to desolventization. These viscosity stabilizing agents do not react with the siloxane terminal end groups of the polymer, but rather serve to neutralize the by-product lithium compounds in admixture with the polymer, thereby preventing the formation of low boiling alcohols during desolventization at the rate normally produced. Thus, the rate of the hydrolysis reaction and, therefore, the rate of coupling of the polymer which, in turn, correlates to the increase in Mooney viscosity of the siloxane terminated polymer having at least one hydrolyzable substituent on the siloxane end group during contact with water or steam, is slowed substantially. The Mooney viscosity of the polymer can therefore be controlled not only during the desolventization process, but also during subsequent storage of the polymer for a limited period of time where the polymer may be subjected to hydrolysis in the form of moisture in the air or in some other manner.
Other attempts at controlling the Mooney viscosity of siloxane terminated polymers have included the use of alkyl alkoxysilanes such as octyl triethoxysilane as viscosity stabilizing agents. These viscosity stabilizing agents are also added prior to desolventization but, unlike the acids set forth in U.S. Pat. No. 5,659,056, react with the siloxane-terminated polymers. Moreover, these viscosity stabilizing agents, i.e., alkyl alkoxysilanes, have the ability to eliminate, not just slow down, any increase in the Mooney viscosity for a period of time. The successful use of these viscosity stabilizing agents however is extremely concentration dependent. That is, the number of --SiOR groups available from the addition of the viscosity stabilizing agent must be such that any Si--O--Si bonds formed are between the hydrolyzable siloxane terminated polymer and the viscosity stabilizing agent, not between the polymers themselves. Furthermore, alkyl alkoxysilanes are relatively expensive compared to many other materials.
Therefore, the need continues to exist for alternative methods for stabilizing and controlling the rate of increase in Mooney viscosity of siloxane-terminated polymers. Such methods preferably would not be dependent upon the concentration of the added viscosity stabilizing agent and would not affect the pH of the polymer.